The invention is based on a fuel injection device for internal combustion engines, such as a unit fuel injector, as defined hereinafter.
With a fuel injection device of this kind, particularly if it is embodied as a unit fuel injector, a greater degree of freedom in terms of open- and closed-loop control actions in the overall injection process is attained than is possible with conventional injection devices, while maintaining high fuel metering quality. In conventional fuel injection devices, the fuel quantity and injection onset are controlled by edges of the pump piston, with the aid of mechanical or hydraulic speed governors. Moreover, without disadvantage to these open and closed-loop control actions, it is possible with these unit fuel injectors to operate at a higher injection pressure, which is necessary in some high-speed direct-injection engines, because the camshaft of the engine is usually used directly to drive the pump piston. By thus using existing drive mechanisms, the cost and drive losses are furthermore reduced.
An essential feature of these unit fuel injectors is the use of an intermediate piston, which separates a pressure chamber from a pump chamber; these terms are largely arbitrarily selected, because naturally both chambers are high-pressure pumping chambers. The term pump chamber has been selected because this chamber is immediately adjacent to the pump piston, and the term pressure chamber has been selected for the other chamber because the injection cannot begin until a suitable pressure is attained in that chamber. While the volume remaining in the pump chamber when the high pressure for the injection is set serves to determine the injection onset, the fuel volume present at that moment in the pressure chamber is used for injection. By using this kind of intermediate piston, a clear separation between the open and closed-loop control actions for controlling the injection onset or the injection quantity is attainable without entailing considerable additional expense; by intermingling the functions, the tasks of a fuel injection pump and the tasks of a hydraulic governor are attained in one unit. A great number of known unit fuel injectors of this kind, operating with an intermediate piston, are known, as well as a number of corresponding unit fuel injectors not having a later priority date. Depending on the type of control, the intermediate piston may be coupled to the pump piston, leaving a certain free stroke; its stroke can be determined by stops; it may be urged by springs either in the direction of the pump piston or away from it; and it can be used to control hydraulic flows, or in other words arbitrary conduits that discharge into the pump cylinder. However, it is always the same diameter as the pump piston.
The cam used to drive the pump piston and thus the intermediate piston of the unit fuel injector is usually disposed on the engine camshaft, so that the drive and the associated control processes are effected as a function of the rotational angle, and the resultant control times are directly rpm-dependent. The drive path of the cam is divided into three segments: a steep compression stroke segment (compression stroke edge or curve), an ensuing, slowly descending intake stroke segment (drop edge or curve), and if desired for control, a resting segment (base circle of the cam), which then merges again with the compression stroke segment. The compression stroke segment of the cam path is relatively short and steep, to attain the desired injection effect, which requires a rapid pumping motion of the pump piston and hence of the intermediate piston. The intake stroke segment, contrarily, is embodied relatively flat, so that in combination with the resting segment as much control time is available as possible for metering fuel in the two chambers, because in these unit fuel injectors the fuel metering is also effected by timing control, namely by opening or closing a fuel line by means of the control valve.
Aside from the rotational-angle-dependent control effected with the joint action of the control edges of the pump piston and intermediate piston, and the time-dependent control determined by the control valve, various valves preventing a reverse flow are also involved in the control; of these, the following description will refer only to check valves, although such valves may also include those having no valve seat, for instance being embodied as slide valves.
The actual problems in such control are much greater than can be described here, because of the parasitic oscillations that occur in such systems and can hardly be controlled. Such parasitic oscillations are produced on the one hand at the control valve and on the other even with mechanical pump piston drive, because in the latter case although there is a constant amplitude, the frequency is dependent on the rpm; that is, the parasitic oscillations are of high frequency if the actual compression stroke at high rpm is very brief. The parasitic oscillations can thus be distinguished by their phase relation from the oscillation resulting from the injection. Although such parasitic oscillations that are ascribable to the mechanical drive are virtually uncontrollable, nevertheless a quantitative estimate of their effect on the range of variation in the fuel quantity can be made. In simplified terms, it can be assumed that with control-edge control, the control quantity remains unaffected by the parasitic oscillation. The situation is different with fuel quantity control by means of a time-controlled valve, in which the critical parasitic frequency is very high, and at a constant amplitude can cause an error in fuel quantity of several percent, and this error is not readily controllable. On the other hand, the advantages of such a control valve, especially when it is a magnetic valve that is triggered by an electronic control unit, are indispensible to meet technical open- and closed-loop control requirements. Naturally this is true not only for a magnetic valve but for any corresponding electrically controlled device.
In a known unit fuel injector (U.S. Pat. No. 4,235,347) of this generic type, the supply quantity and supply onset are determined by the opening and closing of a magnetic valve disposed in the metering line to the pump chamber; a filling line unaffected by the magnetic valve and in which a check valve is disposed leads to the pressure chamber of the unit fuel injector. During a first intake stroke section of the pump piston, the magnetic valve remains closed, so that with the now-closed pump chamber the intermediate piston is pulled along as well, as a result of which fuel at low pressure flows via the fill line and the check valve into the pressure chamber. The force of a spring engaging the intermediate piston in the direction of the pressure chamber is overcome in this process as well; in other words, the low pressure downstream of the check valve, or in other words in the pressure chamber must likewise be sufficiently high to overcome the spring force. Once the supply quantity desired for the injection has been metered, the feed line leading to the pump chamber is opened via the magnetic valve, so that during the continuing intake stroke, this fuel at low pressure now flows into the pump chamber and acts upon the intermediate piston on the side of the spring, causing this piston to remain in its position.
In this known unit fuel injector, the magnetic valve then remains opened until the beginning of the subsequent compression stroke, and during the remainder of the intake stroke fuel flows into the pump chamber and continuously fills it. During the resting section of the cam path as well, the magnetic valve remains opened, and then closes only after the beginning of the compression stroke and at a desired instant of injection onset. From this closing instant of the magnetic valve on, the fuel enclosed in the pump chamber effects a drive of the intermediate piston and thus a feeding of fuel out of the pressure chamber to the nozzle.
The control of quantity for the pressure chamber and the pump chamber, that is, the control of the injection quantity and the quantity determining the instant of supply onset, thus takes place in a time-controlled manner via the magnetic valve, and a control process thereof is effected during the pressure stroke, namely in order to initiate the high pressure. While fault deviations of the magnetic valve are less critical during the intake stroke, during the compression stroke they can cause errors in fuel quantity of up to 30%. In either case, this kind of determination of fuel quantity is too imprecise, and besides the above-mentioned control errors, any other possible influences must also be taken into account, such as leaks in lines and valves, changes in flow conditions for instance caused by opening forces of the check valves, and varying temperatures of the fuel or a varying fuel pressure of the feed pump.
A further disadvantage of this purely time-controlled, known unit fuel injector is that especially at relatively high rpm the mass inertia of the intermediate piston and also throttling effects in the feed line and fill line have a disadvantageous effect. A very precise adaptation must for instance be made between the throttling effect of the magnetic valve to the pump chamber and that of the check valve to the pressure chamber, in combination with the work faces of the intermediate piston, the fuel pressure, and the force of the spring urging the intermediate piston toward the pressure chamber Since the dynamic force of the massive intermediate piston varies in accordance with rpm, and this intermediate piston must be stopped for determining the injection quantity by relatively slight forces during its intake stroke, the result is extraordinarily complex function equations, so that it is virtually impossible to provide a theoretical predetermination in this system; instead, the system must be adapted empirically to each engine, and as is well known each engine, even in the same production run, has a different oscillation pattern.
Although the continuous filling of the pump chamber and pressure chamber advantageously suppresses cavitation, the closure of the inflow line to the pump chamber during the pressure stroke, for the reasons given above, causes deviations from the desired set-point injection time, which is particularly disadvantageous if the instant of injection must be adjusted extremely far away from any parameters. This occurs particularly if the load-dependent injection timing adjustment and the rpm-dependent injection timing adjustment are both in the same adjusting direction. To enable this magnetic valve in the known unit fuel injector to withstand the high pressure, it must be suitably embodied. Although there is as yet no injection pressure in the return feeding, this return feed pressure, which is much higher than the feed pump pressure, varies as a function of rpm within certain limits, which likewise has an influence on the closing process, and certainly on the return feeding process, with a tendency to delay the onset of supply toward "late". However, since the magnetic valve must also be designed for the injection pressure, in other words a maximum possible fuel pressure, which is about 100 times higher than the feed pump pressure, not only the production cost but also the current consumption during operation are correspondingly high.
In addition, in terms of purely time-dependent control of the supply quantity, it should be noted that the supply quantities between idling and full load vary at least at a ratio of 1:15, but also up to 1:30, in each case between the lowermost rpm and the maximum rpm, so that a corresponding effect of the throttling action on the fuel quantity control is unavoidable, and this is always associated with quantity control errors. Naturally the throttling action at the check valve at the entry to the pressure chamber, in this known unit fuel injector, is very much less at the lowest rpm and with an idling quantity than at full load quantity and maximum rpm, in which 30 times the quantity, for example, must be pumped through the same cross section in 1/30 the time.
Taking these conditions into account, it is already known (from German Offenlegungsschrift No. 37 00 352), in a fuel injection device and in particular a unit fuel injector of the type described initially to control the feed line, controlled by the control valve, by the pump piston as well, and during a first intake stroke segment of the pump piston to disconnect this feed line from the stroke chamber and connect it with the fill line of the pressure chamber, and during a later segment of the intake stroke, that is, in the resting segment of the intake stroke and in the resting segment of the cam driving the pump piston to re-connect this feed line to the pump chamber, after previously disconnecting it from the fill line. With this combination in terms of quantity control in the pump chamber and in the pressure chamber of angle-dependent control-edge control and time-dependent magnet-valve control, the above-discussed quantity control errors can be cut in half, and the initiation or end of quantity control is defined by a control-edge control. In this way, with acceptable variation of the set-point metering quantity, the use of an electronic control unit, namely to trigger the magnetic valve, is possible, as a result of which an optimization of the injection to engine parameters is attainable in the simplest way. Naturally, instead of a magnetic valve, a valve controlled in some other way may be used.
A particular advantage of that system is that the quantity metering, determining the supply onset and the supply quantity, is done during the intake stroke, or in other words on the return cam edge, and for this purpose this return edge including the resting segment is enlarged, with a corresponding decrease in size of the compression stroke edge. The compression stroke edge now serves merely to provide pure high-pressure injection, and therefore advantageously be embodied very steeply, which is advantageous above all for the course of injection. Since no quantity control now takes place on the high-pressure side, the course of injection can be varied arbitrarily. In accordance with the steep injection course, the rotational angle of the cam used for injection is also relatively small, which favors the rotational angle that is available for the trailing edge or pitch circle of the cam; the drop edge can be made five to seven times flatter than the compression stroke edge of the cam. In accordance with the available rotational angle, more time is correspondingly available for the intake stroke segment and the resting segment, which is particularly advantageous at high rpm. Because of the longer metering time that is available, the critical disturbance frequencies are also lower. It is also possible for the disturbance frequency to approach the vicinity of the injection frequency, which predominantly determines the phase relation and amplitude of an adjacent disturbance frequency. By means of this longer available control time, the variation in the fuel quantity to be controlled, dictated by the magnetic valve, is also less. Because of the available time, the control of the quantity that determines the supply quantity and the supply onset can be done without difficulty with only one magnetic valve. A particular advantage is that the magnetic valve operates only in the low-pressure range, and can be correspondingly simply embodied, despite high switching frequencies.
In this known unit fuel injector, the reversal of the fuel feed line from the pressure chamber to the pump chamber and back again is effected via the pump piston, or a control slide driven in synchronism with the pump piston, so that the beginning and end of metering is connected as a function of rpm. Thus, at the beginning of the intake stroke the control valve is closed, and it opens to initiate feed quantity metering into the pressure chamber, and because of the suction effect of the pump piston, the intermediate piston already executes the necessary stroke. The larger the supply quantity is to be, the earlier the magnet valve opens. In each case, feeding into the pressure chamber is interrupted whenever the pump piston blocks the feed line. The control valve also continues to be open as long as the pump piston keeps the feed line closed, and then upon reopening, the feed line connecting it to the pump chamber by cooperation with the resting segment of the cam, the pump piston assumes its dead center position. The fuel flowing through the control valve (magnetic valve) is accordingly pumped into a void, and the supply onset begins later, the earlier the control valve closes. The intermediate piston itself assumes a floating position in accordance with the negative pressure, which makes it more difficult to control not only the disturbing oscillation but also the magnet-dictated errors in fuel quantity, because mass acceleration forces of the intermediate piston can have an effect on the metering taking place at low pressure.
It has been set forth heretofore in an application Ser. No. 340,210, filed Apr. 19, 1989 in the U.S. Patent and Trademark Office, (German Patent Application No. P 38 23 827.6) to provide an adjusting spring, acting in the direction of the pump chamber, on the intermediate piston, and to provide a stop, determining the outset position before the onset of the compression stroke, for the intermediate piston, so that this intermediate piston will in each case attain its stop during the drop edge of the cam. As a result, above all the peripheral conditions relating to vibration are the same in each cycle. The control valve can open and close again during the supply quantity metering segment, or combined with the rpm-dependent edge control of the pump piston it can determine the onset time or end time of this metering. The remaining voids in the pressure chamber and pump chamber are thus suitably divided up in accordance with the instance of supply and in accordance with the injection quantity. This clear division of the metering segments permits improved monitoring in quantity control, above all of the hydraulic connections, and thus of the disturbance vibration causing the variations. The supply from the feed line takes place in each case in a pressure chamber that is always the same for each cycle, so that the same basic conditions always prevail for metering of the injection quantity.
While a change in the injection quantity is expressed as an rpm variation with the engine load remaining the same, this particular actual rpm is corrected in the direction of the set-point rpm as dictated by the governor, so that taken all in all, variations and errors in the injection quantity control, considered deductively, are eliminated, and so the void remaining as a result of the partial filling of the pressure chamber has virtually no disadvantageous effect.
The situation is different with quantity control into the pump chamber, which determines the supply onset. Errors in the supply onset have an effect on both fuel preparation and fuel combustion, and hence on the noise and exhaust gases produced by the engine. The supply onset must additionally be controlled not only as a function of rpm but also as a function of load, which results in quite complicated governing, and any existing void can be disadvantageous in that case.
With the known unit fuel injector described at the outset above, a void of this kind is avoided by shifting the control of the supply onset into the compression stroke phase, with all the attendant disadvantages thereof.